1. Field of the Invention
The present invention relates to an apparatus such as a digital printer or a digital copying machine for forming an image with use of electrophotographic process with a reversal development system.
2 . Description of Related Art
In the electrophotographic process with a reversal development system, regulations in the process such as the control of the density or the gradation adjustment of an image to be formed are performed in order to get an optimum reproduction of the image in correspondence with the scattering of the characteristics of the photoconductor layer and the toners, the kind of a document to be reproduced or the environment around the apparatus.
The density of an image to be formed with use of electrophotographic process with a reversal development system can be regulated by controlling the grid electric potential V.sub.G of the charger and the bias electric voltage V.sub.B of the development device. The grid electric potential V.sub.G determines the surface electric potential V.sub.o of the photoconductor sensitized by the charger, and then affects the decay electric potential V.sub.I of an electrostatic latent image formed on the photoconductor generated by the exposure. Further, the bias voltage V.sub.B affects the amount of toners carried onto and adhered to the electrostatic latent image from the surface of the development device according to the development electric voltage .vertline.V.sub.B -V.sub.I .vertline..
Then, in this type of an apparatus for forming a digital image, a standard toner image of an intermediate density is formed on the photoconductor by using the exposure of a predetermined quantity of light under a predetermined grid electric potential V.sub.G and bias one V.sub.B, and a sensor is installed in order to sense the reflecting light from the standard toner image so as to detect the amount of the toners adhered. Then, the automatic density control is performed by controlling the grid and bias electric potentials, V.sub.G and V.sub.B, so as to make the maximum density of an image constant automatically according to the value detected by the sensor.
However, there are other points to be considered when the density is controlled automatically by changing the grid and bias electric potentials, V.sub.G and V.sub.B.
The elimination of fog is one of important problems in an apparatus for forming a digital image, especially a full-color digital image. The fog depends basically on the grid and bias electric potentials, V.sub.G and V.sub.B, so that the two electric potentials have to be controlled suitably in order to prevent a fog. However, the elimination of a fog should not affect the automatic density control.
Further, the effect on the gradation correction (so-called gamma correction) has to be considered in case of a half-tone image. In general, the density of an image of a document to be read is not proportional to a density of an image to be reproduced because of the photosensitive characteristics of the photoconductor, the characteristics of toners, the environment (humidity, temperature or the like) to be used and others. This is called the gamma characteristic. Then the gradation correction is needed to correct the quantity-of-light data of exposure beforehand in correspondence with a density data to be read is needed in order to enhance the fidelity of an image to be reproduced. However, if the grid and bias electric potentials V.sub.G and V.sub.B are changed, the gamma characteristics are also affected. Thus, as far as the gamma correction according to the change in the gamma characteristics is not performed, the fidelity of a half-tone image to be reproduced is deteriorated.
In case of a digital full-color image, the color balance of four colors, cyan, megenta, yellow and black, is one of the important problems. The color balance is determined by the read-out characteristics of the image-reading device and the output density characteristics of the printer part. Previously, the color balance correction is performed in the image-reading device. However, the above-mentioned automatic density control by changing V.sub.B and V.sub.G also affects the color balance largely.